Method of producing sheets of crystalline material and devices made therefrom

ABSTRACT

A method of producing sheets of crystalline material is disclosed, as well as devices employing such sheets. In the method, a growth mask is formed upon a substrate and crystalline material is grown at areas of the substrate exposed through the mask and laterally over the surface of the mask to form a sheet of crystalline mate 
     GOVERNMENT SUPPORT 
     Work described herein was supported by the U.S. Air Force.

GOVERNMENT SUPPORT

Work described herein was supported by the U.S. Air Force.

This is a continuation of co-pending application Ser. No. 07/128,732filed on Dec. 4, 1987, now abandoned, which is a continuation of U.S.Ser. No. 06/251,214 filed Apr. 6, 1981 now U.S. Pat. No. 4,727,214 whichis a continuation-in-part of U.S. Pat. No. 06/138,891 filed Apr. 10,1980, now abandoned.

TECHNICAL FIELD

This invention is in the field of materials, and more particularlyrelates to the production of sheets of crystalline material, includingthin sheets of crystalline semiconductor materials grown epitaxially onsingle crystal substrates.

BACKGROUND ART

In many solid state electronic devices, the active volume of the devicecomprises or lies within a thin sheet, film or layer of crystallinesemiconductor material, often in the single crystal or monocrystallineform. This is particularly true of devices or integrated circuits formedfrom semiconductors such as gallium arsenide, silicon, germanium, indiumphosphide, cadmium telluride, etc. Present techniques for fabricatingsuch devices, however, require that the crystalline sheets be formedupon or near the surface of relatively thick substrates of high-purity,single crystal semiconductor material, and the use of such substratesfor each sheet produced tends to inordinately increase the cost ofproducing the thin sheets. The substrate costs come about from manycauses which include the cost of raw materials, purification, crystalgrowth, cutting, polishing and cleaning.

It has been recognized that by employing a reuseable substrate all ofthe above costs could be reduced and many of the costs would beeliminated, and only a minimum of processing cost might be added back.Thus, attempts have been made to employ reuseable substrates in theproduction of thin sheets of single crystal semiconductor materials, andamong these attempts are the following.

Milnes and Feucht have suggested a peeled film technology forfabricating thin films of single crystal silicon. In the suggestedprocedure, a thin sheet of single crystal silicon is prepared bychemical vapor deposition of a thin silicon film on a silicon substratepreviously coated with an epitaxial layer of a silicon-germanium alloy,thus forming a heteroepitaxy structure. The silicon film is thenreleased from the substrate by melting the intermediate layer ofsilicon-germanium and subsequently peeling the silicon film from itssubstrate. The substrate may be reused in such peeled film technology.See Milnes, A. G. and Feucht, D. L., "Peeled Film Technology SolarCells", IEEE Photovoltaic Specialist Conference, p. 338, 1975.

The Milnes and Feucht peeled film technology was subsequently extendedto the production of gallium arsenide solar cells by employing a thinintermediate layer of gallium aluminum arsenide. In this case, theintermediate layer of gallium aluminum arsenide was selectively etchedby hydrofluoric acid and the single crystal thin film of galliumarsenide could then be removed from the substrate, which could bereused. See Konagai, M. and Takahashi, K., "Thin Film GaAlAs-GaAs SolarCells by Peeled Film Technology," Abstract No. 224, J. Electrochem.Soc., Extended Abstracts, Vol. 76-1, May, 1976.

Another technique for using a reuseable substrate to produce thin filmsof single crystal semiconductor materials is disclosed in U.S. Pat. No.4,116,751, issued to Zaromb. In this technique, a continuousintermediate layer is also employed between a monocrystalling substrateand an outer material grown epitaxially to the substrate. The continuousintermediate layer can be broken up by cracking, sublimation, selectivemelting, or other techniques so that the outer layer can be removed fromthe substrate.

Such prior techniques for reusing single crystal substrates to producesheets of single crystal material have suffered from certain inherentproblems. As an example, these prior art techniques necessitated thatthe material chosen for the intermediate layer had very specialproperties. For example, the material employed for the intermediatelayer in these techniques was required to be a different material fromthe substrate material and yet be a material which could be grownepitaxially on the substrate and one which would thereafter allow thesheet to be grown epitaxially on the intermediate layer. This greatlynarrowed the class of candidate materials, but beyond these limitations,the intermediate material also had to have melting, sublimation,mechanical, etching or other properties significantly different fromthose of the substrate and overgrown film. Further, the epitaxial growthprocedures required to produce the required heterostructures were foundto be difficult to carry out, which further limited the application ofsuch concepts as peeled film technology. Those procedures employingsublimation or melting of the intermediate layer to separate the filmfrom the substrate required elevated temperatures in processing, andsuch elevated temperatures often had deleterious effects on the devicebeing fabricated.

Techniques employing selective etching were particularly difficult toperform on a practical basis. Since the intermediate layer wasrelatively thin and continuous, it was found to be difficult tocirculate an etchant through the small openings formed at the edges ofsubstrates having films thereon, especially over the large distancesrequired to produce large area sheets. As noted, the preferentialetching properties required for the material of the intermediate layerproduced further restraints on materials which could be selected forthis layer.

As a result, previously suggested approaches to using reuseablesubstrates were found to be impractical for the production of sheets ofcrystalline material, particularly large area, thin sheets ofsemiconductor material, at competitive costs. For any particularsemiconductor material, there was an extremely narrow class of materialswhich could be chosen for the intermediate layer required and theepitaxial growth techniques required to form heterostructures weredifficult to carry out. Because of such problems, these techniques neverachieved general acceptance for the production of crystalline sheets ofsemiconductor material.

DISCLOSURE OF THE INVENTION

This invention relates to the production of sheets of crystallinematerial, particularly thin sheets of crystalline semiconductor materialgrown epitaxially on single crystal substrates which can be optionallyreuseable.

In one embodiment of this invention, a growth mask is formed on acrystallization substrate and crystalline material is then grown atareas of the crystallization substrate which are exposed through thegrowth mask. Growth conditions are selected and employed so thatcrystalline material grows up through exposed areas of the mask and thenlaterally out over the surface of the mask. Deposition and growth ofcrystalline material are continued, allowing further growth, especiallyin the lateral direction, to thereby form a sheet of crystallinematerial over the surface of the mask and crystallization substrate.When the sheet has reached the desired dimensions, growth isdiscontinued and the sheet of crystalline material is separated from itssubstrate, which can optionally be reused. Separation can be achieved byemploying a variety of techniques, including the use of mechanical shockfronts to cleave the sheet of crystalline semiconductor material along acleavage plane.

In an alternative embodiment, lateral overgrowth of sheets ofcrystalline material is achieved without the necessity for a growthmask. This embodiment employs a substrate having thin strips ofcrystalline material on the surface thereof or exposed areas of embeddedcrystalline material. Further growth can then occur laterally from thesestrips or areas to form sheets of crystalline material.

In still other embodiments, various devices are fabricated withoutseparating the sheets of crystalline material from the substrate.

The method for producing sheets of crystalline material on reuseablesubstrates described herein has advantages over previously suggestedmethods for employing reuseable substrates in such production. One suchadvantage is the elimination of the requirement to employ heteroepitaxytechniques to produce an intermediate layer. A much wider class ofmaterials can be employed as suitable growth masks according to thisinvention than the class suitable for the intermediate layers of theprior art techniques. The material employed for the growth mask, forexample, need not be single crystal, nor even crystalline. Further, thegrowth mask need not be grown epitaxially to the substrate, and thereare, in fact, a wide variety of methods for applying the growth maskwhich would not be possible for intermediate layers of the prior art.

A further advantage is that the crystal quality for laterally overgrowncrystalline layers according to this invention can be superior in mostcases to crystalline layers grown by heteroepitaxy techniques.

A still further advantage is obtained because there are no continuousintermediate layers required and separation is more easily achievedbecause of the discontinuous nature of areas of attachment between thesubstrate and overgrown film. Techniques such as cleavage are thus madepossible. On the other hand, it is still possible to employ preferentialmelting, sublimation or etching techniques, if desired, with or withoutheteroepitaxy structures. Even where such heteroepitaxy techniques areemployed, the selective melting, sublimation or etching can be expectedto be more easily carried out because the areas of contact between filmand substrate are discontinuous and have only relatively smallquantities of material which need to be selectively melted, sublimed oretched for separation. In short, the use of the lateral overgrowthtechniques of this invention makes the growth and separation of sheetsof crystalline material on reuseable substrates practical.

It is known that thin (e.g., about 50 μm) silicon solar cells have greatpotential for both space and terrestrial applications. Unfortunately,the present technology for preparing such thin silicon sheets includeswasteful etching and polishing steps, is time consuming and also oftenproduces a wavy surface finish. See Ho, F. and Iles, P. A., 13th IEEEPhotovoltaic Specialists Conference, Washington, D. C. 1978, p 454. Onthe other hand, the invention described herein can bypass these problemsin solar cell fabrication. Continuous silicon layers, about 50 μm thick,can be deposited on silicon substrates with a growth mask configurationthereon. The silicon layers can then be separated, as discussed below,to produce thin silicon sheets without the necessity of the conventionalsteps of crystal cutting, wafer polishing and etching.

In addition to those advantages which result from reusing substrates inthe production of thin semiconductor sheets, there are advantages to theinvention described herein even when the crystallization substrate isnot reused.

For example, it is desirable for the manufacture of integrated circuitsthat thin sheets of semiconductor (e.g., about 1 μm thick) be providedon a low-loss insulating substrate. One example of such a coimbinationis silicon on sapphire (SOS) which is currently widely used forintegrated circuits. The silicon film in SOS is known to have manydefects and low lifetime, whereas the separated films described in thisinvention are likely to be of much higher quality, and will also allow amuch wider combination of substrates and semiconductors. Since thematerial cost is often not a major factor in the cost of an integratedcircuit, the substrate might not be reused for this application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram for one embodiment of a processaccording to this invention;

FIGS. 2a-2f present a series of schematic views illustrating theproduction of a thin sheet of crystalline material on a reuseablecrystallization substrate according to the embodiment of FIG. 1;

FIGS. 3a to 3b present a series of schematic views illustrating a priorart technique for breaking a film from a reuseable substrate;

FIGS. 4a to 4c present a series of schematic views illustrating atechnique of this invention, in simplified form, for cleaving alaterally overgrown film from its crystallization substrate;

FIGS. 5a and 5b are an exploded partial cross-sectional view of thecleavage area of the separated film and substrate of FIG. 4;

FIGS. 6a to 6d present a series of schematic views illustrating in moredetail a specific separation of a sheet of crystalline material from itscrystallization substrate;

FIGS. 7-9 each schematically illustrate a partially separatedcrystalline sheet grown over a variety of crystallization growth mask;

FIGS. 10a to 10c are a series of schematic views illustrating the use ofan adhesion-promoting layer for the crystallization growth mask;

FIGS. 11a and 11b are a series of views illustrating schematicallyanother embodiment of a suitable crystallization growth mask;

FIG. 12 is a process flow diagram illustrating an alternative embodimentof a process according to this invention;

FIGS. 13a to 13g present a series of schematic views illustrating theprocess of FIG. 12;

FIG. 14 is a process flow diagram illustrating processes according tothis invention employing selective etching of the growth mask forseparation of laterally overgrown sheets from crystallizationsubstrates;

FIGS. 15a to 15e, 16a to 16c, 17a to 17c, and 18a and 18b each present aseries of views schematically illustrating a variety of techniques forseparating laterally overgrown crystalline films from crystallizationsubstrates by preferential etching;

FIG. 19 is a process flow diagram illustrating another alternativeembodiment of a process according to this invention;

FIGS. 20a to 20j present a series of schematic views illustrating theprocess of FIG. 17;

FIGS. 21a to 21g and 22a to 22d each presents a series of schematicviews illustrating techniques for forming low-adhesion, easily cleavablegrowth masks for use with this invention;

FIG. 23 is a schematic sectional view of a reuseable substrate which isnot entirely formed from single crystal material but which is suitablefor growing sheets which are substantially single crystal;

FIG. 24 is a schematic view illustrating the use of a laser tocrystallize an amorphous film of semiconductor material;

FIG. 25 is a schematic view illustrating one embodiment of a graphiteheating system suitable for use with this invention; FIG. 25A is anexpanded view of a a portion of FIG. 25;

FIGS. 26a to 26d present a series of schematic vies illustrating the wayin which laterally overgrown films form;

FIGS. 27a and 27b present a series of schematic views illustrating thejoining of discontinuous laterally overgrown films;

FIGS. 28a, 28b, 29a and 29b each present a series of schematic viewsillustrating the formation of potential dislocations in laterallyovergrown films;

FIG. 30 is a schematic illustration of the FAN mask pattern employed todetermine the effect of slit orientation on lateral overgrowth for agiven set of growth conditions;

FIGS. 31-38 are schematic views illustrating lateral overgrowth ofcrystalline films at different slit orientations in the crystal growthmask;

FIGS. 39a to 39c presents a series of schematic views illustrating thelateral overgrowth of films under conditions suitable for creating voidsin the overgrown film.

FIG. 40 is a cross-sectional view of a gallium arsenide solar cell basedupon a laterally overgrown film of this invention;

FIGS. 41a to 41d present a series of schematic views illustrating doubletransfer of a laterally overgrown film;

FIG. 42 is a schematic diagram illustrating a three-cell photovoltaicdevice wherein the cells are based upon laterally overgrown films ofthis invention;

FIGS. 43a to 43d present a series of schematic views illustrating thelateral overgrowth of sheets of crystalline material on substrateshaving thin strips of crystalline material thereon;

FIGS. 44a and 44b present a series of schematic views illustratinglateral overgrowth of sheets of crystalline material from strips ofcrystalline material embedded in a substrate;

FIG. 45 is a schematic view of a reuseable master panel for formingsolar panels according to this invention;

FIG. 46 is a schematic illustration of a process for forming solarpanels according to this invention; and,

FIGS. 47 and 48 illustrate a solar cell fabricated by the techniquesdescribed herein; FIG. 47A is an expanded view of a portion of FIG. 47.

BEST MODE OF CARRYING OUT THE INVENTION

A variety of specific embodiments of this invention will now beillustrated with reference to the Figures. In these Figures, likeelements have been given like numerals.

FIG. 1 is a process flow sheet presenting the steps for one embodimentof this invention. In the first step of this process, a crystal growthmask is formed on a crystallization substrate to cover portions of thesubstrate and to leave a pattern of exposed substrate areas. Thecrystallization substrate can be a single crystal substrate, such as asingle crystal of gallium arsenide or other semiconductor, or any othersubstrate capable of supporting crystal growth on at least some exposedarea of the surface. The growth mask is formed from a material whichwill not support crystal growth under the growth conditions employed, orwill only support slight crystal growth which is not significantcompared to the crystal growth rate at areas of substrate exposedthrough the mask.

In the next step of this embodiment, crystalline material is depositedat exposed areas of the substrate. This might be bone, for example, byplacing the masked substrate in a crystal growth reactor, such as avapor-phase epitaxy reactor. Crystalline material deposits at thoseareas of the substrate exposed by the growth mask, and when growthreaches the surface of the mask, under proper growth conditions, furthercrystal growth occurs laterally out over the surface of the mask.

Crystal deposition and growth are then continued so that lateral growthof material from the exposed masked aras continues until a sheet ofcrystalline material having the desired dimensions has formed. Althoughit is not always necessary, in many cases lateral overgrowth iscontinued until a continuous sheet of material has formed from thediscontinuous growth regions formed at exposed areas of substrate.

A sheet of crystalline material can then be separated from the substrateusing a variety of techniques depending on the growth mask which isemployed. For example, if a low adhesion growth mask is used, the sheetcan be cleaved using a mechanical shock front to cleave the sheet at thesupporting ribs which have grown up through exposed areas of the crystalgrowth mask. Alternatively, various additional materials can beinitially crystallized in step 2 at exposed areas of substrate followedby growth of the crystalline sheet material after which the initiallycrystallized materials can be preferentially etched, melted, sublimed,cleaved, or otherwise removed or broken to separate the sheet ofcrystalline material from the crystallization substrate.

The crystallization substrate can then optionally be reused to growadditional sheets of crystalline material. Usually, some cleaning andother minor preparation of the substrate is necessary, although this maynot be required in all cases. Such substrates can be used multiple timesto grow many sheets of thin crystalline material thereby significantlyreducing the cost for such thin sheets of crystalline material.

FIG. 2 presents a series of views which schematically illustrate theproduction of a continuous thin sheet of crystalline material on acrystallization substrate according to the process of FIG. 1.

In FIG. 2A, relatively thick reuseable crystallization substrate 10 isshown. This substrate can be any material capable of supporting crystalgrowth thereon. As a typical example of a substrate suitable for growingsingle crystal gallium arsenide thereon, reuseable substrate 10 might bea slab of gallium arsenide in the range of 5-50 mils thick and might bedoped or undoped. If the crystalline film produced is to be separated bycleavage, it is preferred, although not essential, that substrate 10have an orientation so that the surface of substrate 10 lies in a planewhich is a preferential cleavage plane for the substrate material.

Crystal growth mask 12 is then applied to substrate 10 and mask 12 has apattern of openings through which substrate 10 is exposed. One typicalpattern found to be suitable is a pattern of slits 14 as shown in FIG.2B. The ratio of width to spacing for slits 14 can be widely varieddepending upon the materials, growth conditions, required layerthickness, separation techniques employed, etc., and the optimum ratio,which depends on the particular application, can be determined bymethods described in more detail below. In general, the width of slits14 is preferably less than the thickness of the film to be grown. Ofcourse, growth masks having patterns of exposed areas other than slitscan also be employed.

One suitable technique for forming crystal growth mask 12 is byemploying carbonized photoresist because carbon has very low adhesion togallium arsenide and is also inert to the reactants normally found in anepitaxial growth system. Hence, carbon is an outstanding material to usefor the growth mask 12 in many applications.

Photoresist, which typically contains carbon, hydrogen, sulphur and/oroxygen, is initially deposited over a crystallization substrate and isthen partially oxidized and volatilized by a high temperature bake toremove the hydrogen, sulphur and/or oxygen atoms. This leaves behind amask of carbon. Such a "carbonization" process provides a way to apply afilm of carbon to the surface of another material by first coating thematerial in a conventional manner with a photoresist, such as Shipley1350J. The photoresist can then be patterned optically usingconventional techniques or can be left unpatterned. Duringcarbonization, the thickness of the film is reduced as the atoms otherthan carbon are volatilized or burned up. Heating in air at 400° C. for1 minute is a typical high-temperature bake necessary to carbonizeShipley 1350J photoresist.

Although there are other techniques for applying carbon layers tosurfaces of other materials, such as vacuum evaporation, sputtering,pyrolytic deposition, etc., these suffer certain disadvantages when usedfor deposition of carbon. For example, films produced by such techniquesare typically high stress, low-adhesion films and further require anumber of steps to pattern. The carbonized photoresist offers severalimprovements over such methods. For example, it typically produces a"mask" which has better adhesion, lower stress, and can be easilypatterned directly with fewer processing steps compared to filmsproduced by prior methods.

Of course, materials other than photoresist can be employed in acarbonization process. The only requirement would appear to be that thematerial be composed of carbon and at least one other type of atom sothat it could be "carbonized" by volatilizing the atoms other thancarbon thereby leaving behind a mask of carbon.

Once the mask is in place on a substrate, crystalline material can bedeposited by placing the masked substrate in a crystallization reactorsystem, or by other known growth techniques. An example of a suitablecrystallization reactor system for epitaxial growth of gallium arsenideto a single crystal gallium arsenide substrate is an AsCl₃ -Ga-H₂vapor-phase epitaxy system. As illustrated in FIG. 2C, crystal growthinitially occurs at those areas of the substrate exposed at the bottomof slits 14 in growth mask 12. This forms ribs 16 within slits 14, whichoften serve as the point at which a completed sheet of crystallinematerial can be separated from its substrate. The height of ribs 16,which is equal to the mask thickness, has been exaggerated forillustrative purposes. Growth continues up through slits 14 and underproper growth conditions thereafter laterally over the surface of mask12 to form laterally overgrown sheets 18a.

In FIG. 2D, continued growth of crystalline material is illustratedresulting in further lateral overgrowth to form sheets 18b, which arethicker and larger in area than sheets 18a.

Deposition and growth can be continued, if desired, until sheets 18bjoin to form a continuous sheet 18 of crystalline material asillustrated in FIG. 2E. In a typical lateral overgrowth of a galliumarsenide film employing a mask with slits having a width of 2.5 μm on 50μm centers, the film thickness might be about 1 μm at the point where acontinuous film or sheet is formed. Growth can be continued, of course,to further thicken the film.

After the laterally overgrown film has been completed, the next step isseparation of this film from its crystallization substrate. The specificseparation technique used is often closely tied to the crystal growthmask employed as well as other growth parameters. FIG. 2F illustratesone possible technique in which separation is obtained by cleavinglaterally overgrown film 18 from reuseable substrate 10 along ribs 16.The flexing of substrate 10 and film 12 have been exaggerated forpurposes of illustration. Cleavage techniques for separation aredescribed in more detail below.

Nevertheless, prior to further discussion of the cleavage technique ofthis invention, a short discussion of a prior art separation techniqueis given. In the cracking techniques previously proposed for use withpeeled film techniques, separation was supposed to occur in theintermediate layer which is a different material from the film. However,since this layer was part of an integral crystal structure whichincluded a substrate, intermediate layer, and film to be peeled, therewas not a well defined plane along which the crystal could break. As canbe seen in FIG. 3A, the line of breakage was supposed to followintermediate layer 20, but usually wandered, as shown in FIG. 3B,thereby creating film 21a having a nonuniform thickness. In order toprevent such wandering of the break, the bonding between some of theatoms needs to be significantly weakened where separation is desired.

With the growth mask described herein, a weak layer in the crystal canbe created. A structure with an embedded growth mask 12a is illustratedin FIG. 4A. In many cases, mask 12a can be formed from a material havinglow adhesion to the crystal, in which case the crystal can be imaginedas having voids wherever mask material occurs, as illustrated in FIG.4B. These tunnel-shaped voids can be thought of as an artificiallyformed cleavage plane since the bonding forces between some of the atomsin this plane have been weakened.

When a separating force is applied, as in FIG. 4C, the crystallinematerial will tend to break at its weakest point, which is the plane ofthe mask 12a. The plane of the break wanders generally within the rangeof the rib thickness, as shown in FIGS. 5A and 5B which are explodedpartial cross-sectional views of the plane of breakage.

With low adhesion in the areas of the growth mask, the forces requiredto separate the overgrown sheet are applied essentially to the ribswhich grow up through exposed areas of the substrate. The larger theratio of the space a between ribs to the rib width b, the more theapplied separation force becomes concentrated at the ribs, which is anadvantage in many cases. However, all that is required is a weakening ofthe crystalline material in one plane to guide cleavage.

In FIG. 5A, cleavage is illustrated along a natural cleavage planethrough the ribs as well as along the artificially created cleavageplane at mask areas. As noted above, the natural cleavage plane is onewhere the average bonding strength between atoms is weaker than at otherplanes. Taking advantage of a natural cleavage plane lessens theseparation forces required and tends to confine cleavage to a narrowband, as shown.

In FIG. 5B, cleavage is illustrated where the artificially createdcleavage plane does not coincide with a natural cleavage plane. Theresult is that cleavage still follows the artificially created cleavageplane, but does not stay confined in such a narrow band as in FIG. 5A.

There are a number of ways in which a mask can be used to create lowadhesion between the substrate and laterally overgrown film. In somecases, a material having the property of inherently low adhesion to thesubstrate and overgrown film materials can be employed as the growthmask. For example, a carbon film is an example of a material which haslow adhesion to gallium arsenide. Since carbon also has good chemicalinertness, as well as the capability to inhibit nucleation duringcrystal growth, it is suitable material for the growth mask when thesubstrate and laterally overgrown film material are gallium arsenide.

A specific technique for cleaving a sheet of laterally overgrownmaterial, such as that produced by the process illustrated in FIGS. 1and 2, is illustrated in FIG. 6.

In FIG. 6A, reuseable substrate 10 has laterally overgrown crystallinesheet 18 thereon, and is positioned in an inverted position to thatshown in FIG. 2E. Crystalline film 18 is initially bonded to a newsubstrate 22, such as a sheet of glass, by a bonding agent, such asepoxy. Sheets of ceramics, metals or other materials can be used for thenew substrate, of course, and other bonding agents can be used as well.

In the next step, a bonding agent, such as wax, is used to bond the newsubstrate 22 to a thicker supporting plate 24, as shown in FIG. 6B.Supporting plate 24 is both thicker and more rigid than new substrate22, but can also be made of glass, as well as metal, etc. It is used toprevent the new substrate from flexing excessively during separation.

In FIG. 6C, another supporting plate 24 has been bonded to reuseablesubstrate 10 and serves a similar purpose as the first supporting plate24.

The cleaving procedure is illustrated in FIG. 6D. Therein, it can beseen that splitting wedge 26, which might be the tip of a screwdriver,for example, is inserted between supporting plates 24 and then gentlydriven inwardly. This creates a shock wave sufficient to make a cleanseparation between reuseable substrate 10 and laterally overgrowncrystalline sheet 18. Separation occurs along the cleavage plane of thecrystalline material.

In some separations, the surface of the reuseable substrate remainspartially covered with growth mask. In such cases, the substrate can beprepared for reuse by removing it from its supporting plate, cleaningoff the residual wax and by removing the remaining growth mask. At thispoint, the substrate can be optionally reused to produce another sheetof crystalline material by forming another growth mask thereon.

On the other hand, where the growth mask has good adhesion to thecrystallization substrate and relatively poorer adhesion to thecrystalline film, the separation can result in the substrate having analmost complete growth mask thereon, in which case it can then bedirectly reused.

FIG. 7 illustrates another embodiment of crystal growth mask forproviding low adhesion between the substrate and the laterally overgrownsheet. As shown, a crystal growth mask has been formed in FIG. 7 from afine grained powder, such as silica powder, which has been formed into apowdery film 30. Powdery film 30 would, for example, provide a suitablegrowth mask for crystalline silicon growth. The individual silicagrains, with special preparation, can adhere to each other in a mannersufficient to keep them in place during lateral film overgrowth.However, the tensile strength of powdery film 30 is low compared to thestrength of crystalline film 18, which facilitates separation.

When a separation force is applied, separation occurs between theindividual powder grains. Such grains can be removed from laterallyovergrown film 18 and from the surface of reuseable substrate 10,typically by etching.

FIG. 8 illustrates another type of growth mask having low adhesion tocrystallization substrate 10 and laterally overgrown sheet 18. In thiscase, growth mask 32 is formed from a material which promotes thecreation of voids under film 18. The voids could be created by having amask with a rough or porous surface, which might be formed from frittedglass. The cleaving forces required to separate laterally overgrownsheet 18 from the reuseable substrate 10 and the growth mask 32 arereduced because of the small contact area therebetween.

FIG. 9 illustrates yet another embodiment of a growth mask 34 with voidsto provide minimum contact between reuseable substrate 10 and overgrownfilm 18. In this case, the voids are created by scalloping the surfaceof reuseable substrate 10 and then employing a mask 34, such as oneformed from silicon dioxide, to cover all but the top surface of thepeaks. Since the area of contact between laterally overgrown film 18 andsubstrate 10 is small, mask 34 need not itself have the property of lowadhesion. Lateral growth occurs from exposed areas of the peaks formingthe structure illustrated in FIG. 9. When separation forces are applied,they are concentrated at the peaks where there is contact betweensubstrate 10 and overgrown film 18.

FIG. 10 illustrates schematically another series of steps in a differentembodiment of this invention. In FIG. 10A, a reuseable substrate 10,such as single crystal gallium arsenide, is first coated with anadhesion-promoting layer 36, such as amorphous or crystalline silicon.This can be a very thin layer, such as only a few hundred Å thick.

Thereafter, a growth mask 12, such as one made of carbonizedphotoresist, is applied as illustrated in FIG. 10B in a fashion similarto that previously described.

FIG. 10C illustrates the next step, which is the etching of theadhesion-promoting layer 36, which might be done by a CF₄ plasma iflayer 36 is formed from silicon. A silicon adhesion-promoting layer 36has good adhesion to gallium arsenide and has been found to form siliconcarbide at the high temperatures employed during epitaxial growth of acrystalline gallium arsenide sheets. The silicon carbide forms a bondinglayer between the substrate and remaining carbon. Subsequent processingcan be carried out as previously described, except that theadhesion-promoting layer 36 tends to keep growth mask 12 in place duringseparation so that subsequent cleanup and reapplication of the mask arenot required prior to reusing substrate 10.

In FIG. 11, an alternative embodiment of a suitable growth mask isformed on the substrate as follows. Very finely divided fused silica isadded to a photoresist which is then applied in the desired pattern byphotolithographic procedures to produce a mask 38a as shown in FIG. 11A.A high temperature bake is then employed to burn away the photoresist. Atypical temperature for this bake is about 600° C. During the hightemperature bake, the photoresist is completely burned away, includingthe carbon. This leaves behind a growth mask 38b formed from fusedsilica powder as shown in FIG. 11B. Although the top layer of particlesmay have good adhesion to the overgrown films, the individual particleswithin the fused silica powder do not have strong adhesion to eachother. This makes cleaving along the ribs easy because of the lowstructural integrity of growth mask 38b.

FIGS. 12 and 13 illustrate an alternative embodiment of the process forgrowing crystalline sheets of material on crystallization substratesaccording to this invention. As shown, the initial steps of the processare similar to those illustrated in FIG. 2A-C. Thus, the resultingproduct from FIG. 2C, as shown in FIG. 13A, is reuseable substrate 10having crystal growth mask 12 thereon with regions 18a of some lateralovergrowth of crystalline material. At this point, however, depositionof material is discontinued and no further crystal growth occurs on theoriginal crystallization substrate 10.

Growth mask 12 can be preferentially etched away, as illustrated in FIG.13B, and secondary substrate 40 is attached to regions 18a ofcrystalline material as illustrated in FIG. 13C. Secondary substrate 40can be chosen from a very wide variety of materials, and need not be amaterial which will support crystal growth. Additionally, substrate 40might have one or more coatings thereon, such as a conductive metalcoating capable of forming an ohmic contact with crystalline sheets 18a.

The laterally overgrown regions 18a are then cleaved from reuseablesubstrate 10 as illustrated in FIG. 13D. The result is that secondarysubstrate 40 now has crystalline regions 18a thereon as shown in FIG.13E. These can be used for further growth of regions 18a, or can bedirectly used in certain device applications.

Secondary substrate 40 with sheets 18a is then placed into an epitaxialgrowth reactor wherein growth of crystalline sheets 18a is continued, asillustrated in FIG. 13F, to form sheets 18b and, if desired, continuedto the point where a continuous crystalline film 18 is formed on thesecondary substrate 40, as illustrated in FIG. 13G. As with previousembodiments, deposition and growth can be continued until a desiredthickness for continuous film 18 is achieved. When crystalline sheet 18reaches the desired dimensions, it can be used in device fabrication.

In addition to the cleavage technique previously described forseparation, other separation techniques can be employed. One of theseadditional separation techniques is preferential etching, which isillustrated in general in FIG. 14. In such techniques, the propertiesrequired of the crystal growth mask are somewhat different from thoseproperties required if a cleaving technique is to be used forseparation. In general, the material used for the mask, and/or aheteroepitaxy layer employed with the mask, is required to be one whichpreferentially etches compared to the substrate and overgrown film.

Specific preferential etching techniques will now be described in detailin FIGS. 15-18.

FIG. 15 illustrates one technique for separating a laterally overgrowncrystalline sheet from a reuseable substrate by preferential etching. InFIG. 15A, crystallization substrate 10 is illustrated with a crystalgrowth mask 12 thereon.

In FIG. 15B, it can be seen that lateral overgrowth is allowed tocontinue until crystalline sheets 18b approach each other. At thispoint, substrate 10 is removed from the reactor and an etchant for mask12 is introduced through trough 42 formed between crystalline sheets18b. This etches away, preferentially, growth mask 12 leaving elongatedvoids as seen in FIG. 15C. If a continuous sheet is desired, substrate10 can then be placed back into the epitaxial reactor and deposition andcrystal growth resumed to produce continuous laterally overgrown sheet18 of any desired thickness on original reuseable substrate 10, asillustrated in FIG. 15D. The elongated voids make separation relativelyeasy, as shown in FIG. 15E.

In FIG. 16A, crystallization substrate 10 is once again employed. Inthis embodiment, the crystal growth mask 12 consists of two layers ofmaterial 12a, which is resistant to a first etchant, and a further layer12b of material preferentially etchable by first etchant sandwichedbetween layers 12a. As shown in FIG. 16A, lateral overgrowth iscontinued until crystalline sheets 18b almost touch, and is thendiscontinued.

As illustrated in FIG. 16B, the area of etch-resistant layer 12a isremoved from the bottom of trough 42 and first etchant is thenintroduced through trough 42 and into the sandwich structure therebypreferentially etching away layer 12b leaving an elongated void in itsplace.

As illustrated in FIG. 16C, substrate 10 is then placed back in anepitaxial reactor and overgrowth is continued until continuous sheet 18in a desired thickness is completed. A second etchant, capable ofetching the crystalline material at ribs 16, is then introduced into theelongated voids to separate crystalline sheet 18 from reuseablesubstrate 10 and crystal substrate 10 can then be employed to formanother masked substrate as shown in FIG. 16A.

In FIG. 17, heteroepitaxy is employed to provide a preferentiallyetchable area. In FIG. 17A; reuseable substrate 10 is shown with crystalgrowth mask 12 thereon. After the growth mask 12 has been formed,deposition of material 44 different from the substrate material 10, isdeposited at the bottom portion of slits 14 in mask 12. If substrate 10is gallium arsenide, the heteroepitaxy material 44 might be, forexample, gallium aluminum arsenide, which can be deposited byheteroepitaxy techniques. Subsequently, epitaxial deposition ofcrystalline material corresponding to substrate 10 is carried out toform sheets 18b. A first etchant is employed to remove mask 12, FIG.17B, and growth is continued to form continuous sheet 18. Subsequently,a second etchant is introduced to preferentially etch away heteroepitaxymaterial 44 thereby separating film 18 from substrate 10, as shown inFIG. 17C. Even though heteroepitaxy material 44 is shown deposited atthe bottom of the ribs, it could also be formed at other sections of theribs, such as at the top, even with a small amount of overgrowth.

FIG. 18 illustrates another technique for preferentially etching alaterally-overgrown film to separate it from its reuseable substrate. Asillustrated in FIG. 18A, substrate 10 has a scalloped upper surface. Thescalloped upper surface of the substrate is masked at all areas exceptat the very tops of the peaks. At this location, deposition and growthof a material 44 different from the substrate is accomplished byheteroepitaxy techniques as preveously described. Deposition and growthof substrate material are then continued to form laterally overgrownfilm 18.

As illustrated in FIG. 18B, separation can be achieved by introducing apreferential etchant for heteroepitaxial layer 44 into the voids formedbetween reuseable substrate 10 and laterally overgrown film 18. Sincethe amount of heteroepitaxial material 44 which must be etched is smallfor the three methods just described, and because voids are availablefor circulating the etchant, these techniques should be particularlyefficient and practical.

FIGS. 19 and 20 illustrate yet another alternative embodiment of thisinvention for producing laterally overgrown crystalline films.

In FIG. 20A, substrate 10, which once again might be single crystalgallium arsenide, is illustrated.

In the first step of this process, a layer 46 of oxidizable maskingmaterial is applied, as illustrated in FIG. 20B. Layer 46 could also beother materials which can be transformed into preferentially etchablematerials, by nitration, or other techniques. An example of anoxidizable material is silicon, which can be oxidized to silicondioxide, a preferentially etchable material.

In FIG. 20C, the application of a pattern of slits 14 to the oxidizablemask 46 is illustrated. This can be done by photolithographictechniques.

Deposition and growth of crystalline material then commences, FIG. 20D,to form ribs 16, after which mask 46 is oxidized to a certain thickness48, FIG. 20E. In a typical case where mask 46 is silicon, mask 46 mightbe oxidized to a depth of about 500 Å by steam at 700° C.

In FIG. 20F, lateral overgrowth of crystalline material is illustratedas in prior cases. The material grows up through slits 16 of mask 46 andoxidized layer 48 and laterally out over the surface of oxidized layer48 to form sheets 18b.

A preferential etchant for oxidized layer 48 is introduced throughtroughs 42, and after the oxidized layer 48 has been etched away, FIG.20G, the overgrown substrate is placed back in an epitaxial reactor andgrowth is continued to produce continuous sheet 18 of crystallinematerial in a desired thickness, as shown in FIG. 20H.

Separation is illustrated in FIG. 20I employing wedges 26 to provide amechanical shock front capable of cleaving the substrate from theovergrown film at ribs 16.

In FIG. 20J, a subsequent oxidation of another portion of mask 46 isshown. Steps 20F-J can then be repeated to form another separated sheet18 of laterally overgrown crystalline material on substrate 10.

FIG. 21 illustrates another optional technique for forming a resuseablesubstrate for use in this invention. In FIG. 21, substrate 51 is acrystalline semiconductor material, such as single crystal silicon. Lowadhesion masks for silicon, and other similar materials, are sometimesmore difficult to achieve than similar masks with gallium arsenide. Forexample, silicon reacts with carbon at growth temperatures above 800°C., making a carbon mask undesirable under such higher temperatureconditions.

Materials such as Si₃ N₄ and SiO₂, however, are relatively inert tosilicon even at high temperatures. However, conventional pyrolytic orthermal deposition typically have very good adhesion to silicon and manyother types of substrates. FIG. 21 represents schematically a newprocess for producing excellent low-adhesion growth masks on substratessuch as silicon substrate 51.

FIG. 21A illustrates the application of a relatively thin (e.g., 1000 Å)silicon dioxide coating 53 to silicon substrate 51. Silicon dioxidecoating 53 can be applied either pyrolytically or thermally. Itsthickness could range from a few angstroms to thousands of angstroms.

A layer of photoresist is then applied over silicon dioxide coating 53and is then carbonized by heating in air at elevated temperatures toform carbonized photoresist layer 55. This layer might have been appliedin a thickness of about 7000 Å and carbonized at 400° for 1 minute toreduce it to about 3000 Å.

As illustrated in FIG. 21C, another layer of silicon dioxide is thenapplied over carbonized photoresist layer 55. The second silicon dioxidelayer 57 might have a thickness similar to initial silicon dioxide layer53 (e.g., 1000 Å).

FIG. 21D illustrates the application of photoresist layer 59, which ispatterned as desired employing conventional photolithographictechniques. For example, the slit openings described herein for lateralovergrowth might be applied, as illustrated.

After application and patterning of the photoresist layer 59, the threecoatings are etched as follows. Silicon dioxide layer 57 is first etchedwith buffered hydrofluoric acid (FIG. 21D), carbonized photoresist layer55 is etched with a heliumoxygen plasma (FIG. 21E), and silicon dioxidelayer 53 is similarly etched with a buffered hydrofluoric acid solution(FIG. 21F).

Patterned photoresist mask 59 is removed by conventional techniques andthe sample is placed in an oxygen atmosphere and baked at hightemperatures. (e.g., 700° for 45 minutes.) During this high temperaturebake in an oxygen atmosphere, the carbonized photoresist layer 55 isselectively removed (volatilized) from between patterned silicon dioxidelayers 53 and 57. This results in the top silicon dioxide layer 57laying neatly down on top of lower silicon dioxide layer 53 in analigned and loosely bonded relationship. The wafer is now complete witha low-adhesion growth mask and lateral overgrowth can begin. Since theupper and lower silicon dioxide layers, 57 and 53, respectively, areloosely bonded, a plane of weakness for cleaving the laterallyovergrowth film from the substrate is provided.

FIG. 22 illustrates an alternate process for forming a low-adhesiongrowth mask on a crystalline substrate formed from a material such assilicon. The first step in this alternative method is the formation of acarbonized photoresist layer 55 (e.g., 3000 Å) directly on the siliconsubstrate 51. A layer of silicon nitride 61 is then pyrolyticallydeposited over carbonized photoresist 55. Silicon nitride layer 61 mighttypically have a thickness from 500 to 1000 Å. Photoresist 59 is thenapplied over silicon nitride layer 61 and patterned by conventionalphotolithographic techniques to form the desired slit openings (FIG.22C). The silicon nitride layer 61 can be etched using a CF₄ plasma. Thepatterned wafer is then baked at high temperatures in an oxygenatmosphere and the nitride layer 61 lays smoothly down and becomesloosely bonded to silicon substrate 51 (FIG. 22D). A suitable bake mightbe at 700° C. for 45 minutes in oxygen. The loosely bonded siliconnitride layer 61 provides a low-adhesion growth mask having a plane ofweakness for cleavage after a laterally overgrown film is formed fromsubstrate 51.

The essence of the methods for forming low-adhesion growth masksillustrated in FIGS. 21 and 22 is the formation of a three-layersandwich where the middle layer can be preferentially etched, bakedaway, otherwise preferentially removed. The layers should be formed withsufficiently low stress and should be sufficiently thin to allow the twomaterials which are not etched to become uniformly loosely bonded. Thethickness and material properties of the three layers are only limitedto the extent that loose bonding is required and that these berelatively inert to each other and to the crystal growth environment. Awide variety of combinations of materials and thicknesses can beemployed.

An alternative embodiment is illustrated in FIG. 23 which is a schematicview of a substrate which is suitable for formation of substantiallysingle crystal laterally overgrown films even through the substrateitself is not single crystal material. In fact, the substrate can beamorphous, polycrystalline, metal, or some combination. As shown, mask12 is placed over substrate 50. Mask 12 might be any of the materialspreviously described and it could even be composed of the same materialas the substrate. At the open areas left by mask 12, seed material 52,which must be single crystal and oriented, is formed. This might bedone, for example, by cutting strips from a sheet of single crystalmaterial and laying such strips over substrate 50. Further growth ofsingle crystal material will cause seed material 52 to grow upwardly andlaterally outwardly over mask 12 to form a laterally overgrown sheet ofcrystalline material. The film could then be peeled away by techniquesdescribed before.

FIG. 24 is a schematic view of a process employing crystallizationsubstrate 54 having crystal growth mask 12 thereon. Substrate 54 mightbe polycrystalline or amorphous, as long as the areas 52 exposed throughmask 12 are single crystal, and as illustrated, further polycrystallineor amorphous material is deposited over the openings and mask to formsheet 56. Subsequent to deposition, an energy beam, such as from pulsedlaser 58, is used to heat film 56 and to crystallize it. Single crystalgrowth is initiated at areas exposed through the mask by the energy beamand lateral overgrowth occurs. Crystallization of film 56 might also beachieved by heating with a graphite strip heater, other means forheating, or by other crystallization techniques.

FIG. 25 illustrates schematically the use of a graphite strip heateruseful for crystallizing a sheet of material. Slab 53, comprising asubstrate of crystalline Si 54 having a growth mask 12 of SiO₂ thereonwhich is overcoated with amorphous silicon 56 (as shown in cross-sectionin the enlarged insert; FIG. 25A) is placed upon a lower graphite heater65 which heats slab 53 to a temperature close to its melting point.Upper graphite strip heater 63 is then scanned across the top of slab 53to heat the amorphous silicon 56 above its melting point.

In FIG. 25, upper graphite strip heater 63 is illustrated as beingscanned parallel to the long axis of the stripe openings. It could, ofcourse, be scanned in other directions. For example, it has been foundthat excellent results can be obtained when the scanned direction isperpendicular to the long axis of the stripe openings. With suchperpendicular scanning, it is possible to propagate a lateral epitaxialfilm from a single stripe opening.

In addition to employing a scanning graphite heater, other heatingsources, such as a laser or electron beam, could also be employed. It ispreferred to employ a beam having a large aspect ratio, consistent withthe geometry of the stripe openings.

It is possible to use a stationary heating technique, such as pulsedheating from a laser or other source. It is also possible to heat slab56 with a stationary graphite heater having a controlled temperaturegradient in the plane of the sample, simulating a scanning effect.

In techniques described above, it is necessary to have lateralovergrowth of material. For purposes of this invention, lateralovergrowth means that at least 10% of the area of the crystalline sheetproduced has grown laterally out over the surface of a crystallizationsubstrate. In many cases, of course, the lateral growth rate will besufficient to allow lateral growth of much greater than 10% of the totalarea of the crystalline sheet produced.

Preferential lateral growth can be obtained by selection of appropriategrowth conditions, crystallographic orientation of the substrate, andorientation of the slits or other openings in the crystal growth mask.The crystal growth conditions which can be adjusted to providepreferential lateral growth include temperature, flow rates,concentrations, growth time, etc.

In most cases, it is preferable that the ratio of lateral-to-verticalgrowth be at least about one. Ratios of about 25 have actually beenachieved in practice, and it is believed that even higher ratios arepossible under proper growth conditions and with appropriate substrateand epitaxial growth mask opening orientations.

Lateral overgrowth of a single crystal film from an opening isillustrated in FIG. 26. The dots represent atoms in a single crystal.Thus, the Figure represents the cross-section of a hypothetical crystal.The atoms in the substrate are completely ordered, as is expected in asingle crystal. As the crystal grows, atoms are added in the sameordering, first to fill the opening in mask 12, then laterally out overthe surface of mask 12. It should be noticed that the atomic ordering isdependent on the atomic arrangements in the exposed areas, and the areasunder the growth mask have little or no effect. The atoms can besupplied in many ways, as for example, from deposition or fromcrystallization of an amorphous layer. It is the property of crystalsunder a given set of growth conditions that the growth rate variesdepending on the direction it is measured. FIGS. 26B, 26C and 26Dillustrate the same crystal of gallium arsenide as FIG. 26A and grown aspreviously described after 5, 10, and 20 minutes, respectively. If thehorizontal and verticle distance between atoms is 10 Å, then it can beseen from FIG. 26C that the lateral growth rate is 4 Å/min and theverticle growth rate is 2 Å/min. Normally, most crystal growth rates aremuch higher than these values and these are used only for the purpose ofillustration. The lateral-to-verticle growth rate ratio is then G_(l)/G_(v) =2. A simple way then to measure the growth rate ratio is to growthe crystal through a mask for a short period then interrupt the growthand measure L and V as shown in FIG. 26C. The lateral-to-vertical growthrate ratio is then G_(l) /G_(v) =L/V (L/V=2 in the illustration).

If growth is allowed to continue from two adjacent openings, the growthfrom the two openings can merge, as shown in FIG. 27A, for a perfectwell ordered sheet. Further growth in FIG. 27B will, in many cases, tendto smooth out the top surface.

Of course, it is not necessary for overgrown regions to join in order toproduce sheets. It is prefectly permissible, according to thisinvention, to produce a multiplicity of sheets on the surface of acrystallization substrate. In this case, each region of laterallyovergrown crystalline material can form a sheet. For purposes of thisinvention, overgrown crystalline material is considered to be a sheet ifthe area of the largest cross-section thereof parallel to thecrystallization substrate is equal to or greater than the area of thelargest cross-section thereof normal to the substrate.

It is possible, in some cases, that flaws can be formed in theovergrowth where the crystals join under certain conditions as shown inFIGS. 28 and 29. In these Figures, the lateral-to-verticle growth rateratio is illustrated as 5, and therefore the overgrowth extends furtherfrom the mask opening for the same thickness film, compared to thatshown in FIG. 26. With such larger extensions, it is possible in somecases to build up stress in the overgrown layer which may cause effectsas illustrated. Of course, in other cases, ratios of 5 and much higherdo not cause such problems.

In FIG. 28A, the atoms of the overgrown layer are under compression andare not lining up exactly with the atoms below the mask. Consequently,as growth continues and the regions of lateral growth join in FIG. 28B,there will be extra space between atoms which can lead to a crystaldislocation.

Similarly, the lateral growth in FIG. 29A causes the stress in theovergrown layer which causes the left side to curl up slightly. When thegrowth joins as shown in FIG. 29B, the match between crystals will notbe good and dislocations will be created. This type of stress flaw mightbe reduced by having a smaller lateral-to-vertical growth ratio becausethe stress distortions of the crystal may be smaller. In some cases,dislocations and grain boundaries will not be detrimental to certainapplications for sheets of crystalline material produced as describedherein.

In most of the examples given, the lateral overgrowth has been shown tooccur symmetrically from both sides of the slit. The symmetry has beenincluded in the Figures for simplicity and is not a requirement nor isit usually the case nor is it necessarily preferred.

Selective epitaxy is epitaxial growth of a crystal from a crystallinesubstrate which has a masking material with openings of designeddimensions and geometries. The crystal growth initiating from theseselective openings will have a crystalline structure similar to thesubstrate and will grow into a geometric shape based on the crystalorientation, mask opening geometry and orientation, and crystal growthconditions. Because all these conditions effect the final growth, onemust carefully design the geometries, angles, growth conditions, andsubstrate orientation to optimize the final growth to meet the specificrequirements.

The lateral overgrowth process often incorporates the use of parallelopenings where the length of the openings is much greater than thewidth, and a pattern was designed to produce openings in a maskingmaterial which not only has a high length to width ratio, but alsovaries the angle of each opening from 0° to 90° in 1° incrementsproducing what is called a "FAN" pattern as shown in FIG. 30. From thisFAN pattern, one can learn not only the effects of substrateorientation, but also the effects of angle of pattern orientation andthe effects of varying growth conditions.

The use of the FAN pattern can be illustrated for gallium arsenide forwhich it was felt that a very high lateral growth to vertical growthratio would produce excellent surface morphology along with a minimum ofgrowth defects. The optimum conditions were found in the followingmanner.

Three major substrate orientations were used, the [100], [110], [111]B.As used herein, brackets indicate plane orientations indices, andparentheses indicate directions perpendicular to the plane orientationswith the same indices. The actual substrates were misoriented a fewdegrees from the major orientation, which does effect the results.

However, in the interest of simplicity, the substrates are consideredexactly oriented and later a brief discussion will show the effects ofmisorientation. Samples of the three orientations were covered with 1000Å of silicon dioxide and patterned using standard photolithographictechniques to produce the FAN pattern openings in the SiO₂. The FANpattern was exposed on two sections of the surface of the substrate,with the major axis of the pattern rotated 90° from one section to theother. This produced line openings where the surface angle varied from0° to 180°. The three patterned substrates were epitaxially grown on atthe same time. By investigating the growth at each angle of line openingon each type of substrate, the optimum conditions to produce layers ofparticular design could be chosen.

It was found that each opening produced a different epitaxial growth.For instance, on the [110] crystal surface, the ratio oflateral-to-vertical growth went from 1 to 25 as the angle of the lineopening varied from 0° to 60° clockwise from a line in the (110)direction. Continuing clockwise to an angle of 90°, thelateral-to-vertical growth rate ratio decreased. On the other hand, a[100] oriented substrate produced excellent growth at 221/2° from the(110) direction, while the [111] B substrate produced good growth at 15°clockwise from any of the three cleaved faces.

Because it is often preferred to have the natural cleavage plane linedup with the artificially created one, the [110] was chosen for GaAs andthe line openings were oriented 60° from the (110) direction. This angleproduces the maximum lateral-to-vertical growth ratio along with thesmoothest surface.

It must be noted that these conditions apply only for the particularcase of GaAs for a particular set of growth conditions. By using thissame FAN pattern, one can determine the optimum conditions for othermaterials, such as silicon, and indium phosphide, and for GaAs underdifferent growth conditions.

In the case of silicon, a 1:1 lateral-to-vertical growth rate ratioproduced the best results under certain growth conditions. This 1:1ratio was found 45° clockwise from the [110] flat on a [111] orientedsilicon wafer using vapor phase epitaxy growth at 1000° C. with SiCl₄and HCl. Because the [111] plane is an easy cleavage plane for silicon,tests for optimum conditions were performed on this orientation.

The lateral-to-vertical growth rate ratio is an important considerationin the design of the growth mask especially in relation to the desiredthickness of the laterally overgrown sheet of crystalline material. Ithas been found, as noted above, that the lateral growth characteristicsfor gallium arsenide and silicon are a strong function of theorientation of slits relative to the crystal orientation. As discussedabove, the effect of the orientation of the slits can be studied withthe aid of a FAN shaped growth mask shown in FIG. 27. Each line in themask of FIG. 27 is a 2 μm wide slit and successive slits are angled 2°from each other over the range -45° to +45°. The full range of slitangles of 180° may be obtained by making two prints of the mask on asubstrate, one print rotated 90° from the other. After lateral growthover such a mask, sections can be made perpendicular to the wafersurface to examine the cross-sectional shape of the overgrowth. Thistype of study has been made for both gallium arsenide and silicon for avariety of growth conditions, and for masks on the [100], [110] and[111] surfaces. Some examples will be given for gallium arsenide wafersin a AsCl₃ -Ga-H₂ system at a growth temperature of 750° C., a galliumtamperature of 820° C., and a hydrogen flow rate of 900 cc/min in a 54mm ID tube.

FIGS. 31-36 illustrate lateral overgrowth (o) through a 2 μm slitopening where the masked surface is the [100] plane. In FIG. 31, theline opening is along the (110) direction, and in FIG. 32, the lineopening is along the (011) direction. For these cases, thelateral-to-vertical growth rate ratio is small, and for FIG. 31 there iseven an overhang. FIG. 33, 34 and 35 are at +22.5°, +67.5° and 112.5°,respectively, from the (110) direction. The slight tilt of the topsurface in FIGS. 31 and 32 is caused because of the slight (2°)misorientation of the substrate. That is, the surface is not exactly[100] oriented. In FIG. 36, the wafer surface was oriented to the [100]plane and the slit is oriented 60° clockwise from the (110) direction.This orientation gives the largest lateral-to-verticle growth rate ratiounder the previously given growth conditions for the [100] surface.

FIG. 37 gives an example of the type of results obtained from the FANshaped mask pattern for gallium arsenide on the [100] surface. Each slitis oriented 1° from the other, starting with the [110] direction on theleft. The second slit from the left is rotated 1° clockwise from thefirst, the third 2° from the first, and so forth. For this example, thelateral-to-vertical growth rate ratio is increasing as one moves awayfrom the (110) direction.

FIG. 38 illustrates growth through slits for the same time andconditions as those of FIG. 37 at a different substrate orientation. Theslit on the left is the (110) direction, with succeeding slits eachrotated 1° clockwise. It is interesting to note the void 71 enclosed onthe right side which occurs during lateral overgrowth. This void wouldshow up as a groove on the back of a peeled layer, which might be anadvantage or a disadvantage depending on the application.

In FIG. 39, an embodiment is shown where the growth of voids 71 of FIG.38 is used to an advantage. With growth mask 12 in place, FIG. 39A,growth is begun and continued, FIGS. 39B-39D. Voids 71 are formed whichcan serve as a weak point for separating the film by cleaving, FIG. 39E,or they can be used as openings to circulate an etchant.

FIG. 40 illustrates the fabrication of a gallium arsenide solar cellproduced employing the techniques described herein.

A single crystal substrate 10 of gallium arsenide is shown separatedfrom the solar cell formed with a sheet 18 of laterally overgrowngallium arsenide. Any of the previousdly described techniques forlateral overgrowth of film would be suitable. The p⁺, p and n⁺ carrierconcentrations are achieved simply by changing the dopants present inthe epitaxial reactor. The anodic oxide layer, tin contact, transparentepoxy and cover glass elements are all added by known techniques, andparticularly as described in Ser. No. 22,405, filed Mar. 21, 1979, whichdiscloses in great detail the preparation of shallow homojunction solarcells.

Silicon solar cells could be formed by similar techniques. In siliconsolar cells the thickness of the silicon should be at least 20 μm,unless a back surface reflector is employed whereby the thickness mightbe as thin as 10 μm. Such thin layers can be obtained as grown withoutthe requirement for polishing.

Indium phosphide solar cells need active layers of only about 2-3 μm, asis the case with gallium arsenide. With a back surface reflector, 1 μmthick indium phosphide on various secondary substrates would besuitable.

FIG. 41 illustrates double transfer of a laterally overgrown sheet ofcrystalline material. The process begins with the production oflaterally grown epitaxial film 18 ready to be removed from the primarysubstrate 10 in FIG. 41A, as described above. Film 18 is bonded tosecondary substrate 60 and removed from primary substrate 10 as shown inFIG. 41B. Another secondary substrate 62 is bonded to the surface offilm 18 by bonding layer 64 in FIG. 41C. Bonding layer 64 can be epoxyor any other appropriate bonding material. First secondary substrate 60is now removed by preferential etching, melting, etc., of mask 12 asshown in FIG. 41D. The structure in FIG. 41D is now ready forfabrication into devices in much the same manner as would be an ordinarysingle crystal wafer. If the single crystal film in FIG. 41 were galliumarsenide, silicon, or indium phosphide, one could consider making solarcell or integrated circuits, for example.

FIG. 42 illustrates three solar cells fabricated in tandem. This ispossible due to the methods described herein for producing very thinfilms of single crystal semiconductor material. As shown in FIG. 42,bottom cell 70 need not be thin but upper cells 72 and 73 are thin. Thethree cells can be bonded together with transparent insulating epoxywith proper optical matching. The advantage over a heteroepitaxy tandemcell is that the current and voltage of each cell are decoupled, thatis, they can be wired independently. Alternatively, the three cells canbe bound together with transparent-conducting epoxy or a layer such asSn-doped In₂ O₃. In this case, the cells are connected in series. Theadvantage of this over conventional tandem cell approach is that thethree cells can be fabricated separately and subsequently bondedtogether. In the conventional tandem cell approach, the three cells mustbe grown monolithically, with all the inherent difficulties, such aslattice-matching, material interdiffusion, etc. Of course, any number ofcells can be joined in tandem.

FIG. 43 illustrates lateral overgrowth without a mask. Substrate 80 canbe single crystal, polycrystalline, amorphous, metal, insulatingmaterial, etc. The only property required of substrate 80 is that itallow lateral overgrowth, as previously described, and that it remainrelatively inert during lateral overgrowth upon its surface. It might bean advantage in some cases to choose a material for substrate 80 whichhas low adhesion to the laterally overgrown film produced thereon. Ofcourse, a material which does not have the desired properties might beprovided with a film at its upper surface to provide such properties andsuch a coated substrate would also be suitable.

In FIG. 43B, strips 82a, preferably of single crystal material, havebeen placed or bonded upon the surface of substrate 80. These will serveas suitable sites for crystal nucleation and growth during the lateralovergrowth process.

In FIG. 43C, lateral overgrowth has begun to widen strips 82a intosheets 82b. Lateral overgrowth can be continued as previously describedin this application to produce a sheet of crystalline material ofdesired dimensions.

In FIG. 43D, for example, lateral overgrowth has occurred to the pointwhere continuous film 82 has been formed on the surface of substrate 80.Continuous film 82 can now be separated by any of the techniquespreviously described herein. In particular, if the surface of substrate80 has low adhesion to film 82, it might be possible to simply lift offfilm 82.

FIG. 44 presents a series of schematic views illustrating the embeddingof strips of single crystal material into a substrate. In this case,substrate 84 need not be single crystal and can, in fact, be formed fromany material upon which significant crystal nucleation will not occurduring a lateral overgrowth technique. Embedded strips 85 of singlecrystal material might be produced, for example, by a technique asillustrated above with regard to FIGS. 12 and 13.

Substrate 84, with embedded single crystal strips 85, can then be placedin an epitaxial growth reactor and lateral overgrowth can be commencedto form overgrown areas 85a.

Of course, strips 82a and 85a, shown respectively in FIGS. 43 and 44,need not be in the shape of elongated strips and could be a series ofsegmented strips or could, for that matter, have other shapes.

FIG. 45 illustrates schematically a largearea solar cell panel whichcould be manufactured using the methods described herein. Reuseablemaster panel 90 might have a size, for example, of about 2'×4'.Presently, there has been great difficulty, if not impossibility, infabricating semiconductor sheets in such a size. In this case, reuseablemaster panel 90 is formed from a plurality of insertable smaller units91 which are cemented in a contacting and aligned relationship to asuitable substrate 92, which might be a ceramic plate. Inserts 91 couldbe formed in the size of about 6"×12" and might be formed from germaniumsubstrates having a thin film of single crystal gallium arsenide on thetop surface.

One method for using reuseable master panels 90 in mass production ofsolar cell panels is schematically illustrated in FIG. 46. A galliumarsenide vapor-phase epitaxy reactor capable of holding many reuseablemaster panels is provided. Gallium arsenide lateral epitaxial films aregrown in this reactor over reuseable master panel 90 as follows.

Initially, a crystallization growth mask is applied to the singlecrystal gallium arsenide layer on film inserts 91. This mask might becapable of repeated cycles through the reactor. In the reactor, alaterally grown gallium arsenide film is grown up through the mask andover the surfaces of inserts 91 to form a continuous gallium arsenidefilm over panel 90.

The panels 90 are removed from the reactor and solar cells are thenfabricated by traditional steps, such as plating, anodization, etc., onthe surface of each film insert 91. The gallium arsenide laterally grownepitaxial film containing the cells is then bonded to a glass platehaving a size similar to the reuseable master panel 90. Gallium arsenidecells are transferred to the glass sheet from master panel 90 bycleaving along a plane of weakness as described above. The reuseablemaster panel 90 can then be recycled through the reactor. The epitaxialfilm on the glass substrate requires a few additional steps to completefabrication of a solar panel. Clearly, lateral epitaxial films of othersemiconductor materials could be employed in the fabrication of suchsolar panels.

This invention can be more specifically illustrated by the followingexamples.

EXAMPLE 1

A single crystal gallium arsenide substrate, 15 mils thick, and dopedwith chromium to make it an insulator was employed. The substrate wasoriented in the [110] direction, which is a cleavage plane. Shipley1350J Photoresist was spun onto the surface and dried in a pre-exposurebake. A pattern of stripe openings, each 2.5 μm wide, each on 50 μmcenters was contact printed onto the photoresist oriented 60° clockwisefrom the [110] plane. The coated substrate was then heated to 400° C. inair for one minute to carbonize the photoresist.

After brief chemically etching, the coated substrate was placed in anAsCl₃ -Ga-H₂ epitaxial reactor and the substrate was heated to atemperature of 740° C. Crystal growth occurred in the stripe openingsthrough the carbon layer and subsequently out over the surface of thecarbon layer. The lateral growth from adjacent openings joined when thethickness of the film was approximately 1 μm. Growth was continued untilthis continuous film has a thickness of 5.8 μm.

In order to transfer the laterally-grown film from its originalsubstrate, the surface was bonded to a 10 mil thick glass plateemploying Hysol white epoxy-patch kit, Number 0151. This epoxy is rigidand contains no bubbles. The epoxied sandwich was bonded between tworigid glass plates using wax. In order to separate the film from thesubstrate, the tip of a screwdriver was inserted into the gap betweenthe two plates and then tapped lightly with a hammer. The layersseparated easily along the [110] cleavage plane where GaAs grew upthrough the carbonized mask because of the low adhesion of carbon togallium arsenide and crystal weakness. Thus, the entire epitaxial filmwas bonded to its new substrate while the original substrate remainedattached to the glass plate.

The substrate was removed from the glass plate and cleaned with adetergent spray to remove the carbon film after which it was in nearlythe same condition as at the beginning of the process. Approximately1,000 Å of gallium arsenide was then removed from the substrate using alight cleaning etch and the substrate was reusable.

EXAMPLE 2

The procedures and materials of Example 1 were employed, except asotherwise noted, to produce four films of gallium arsenide on glasssubstrates using the same substrate. These films were 5, 10, 10 and 8μm, and the area of each film was about 3.8 cm. From these results, itwas estimated that at least 1000 films could be generated from one 25mil thick gallium arsenide substrate.

EXAMPLE 3

The electrical characteristics of a film produced according to thisinvention were evaluated as follows. An epitaxial, sulphur-doped layerwas grown on two wafers, one with preparation for the process employingreuseable substrates described herein, and the other as a control samplewithout any photoresist masking. Both were chromium doped,semi-insulating substrates with [110] orientation. After growth, Hallmeasurements were made on both wafers using the Van der Pau techniquewith ohmic contacts at the corners of one-quarter cm² Hall samples. Theepitaxial film on the reuseable substrate sample was transferred to aglass substrate with contacts and wires still attached. Measurements ofthis transferred film were then made again and the results of allmeasurements are presented in Table 1.

                                      TABLE I                                     __________________________________________________________________________                       Thickness                                                                           Temperature                                                                          Electron Density                                                                       Electron Mobility                    Sample             (μm)                                                                             (°K.)                                                                         (e/cm.sup.3)                                                                           (cm.sup.2 /V-sec)                                                                      N.sub.A /N.sub.D            __________________________________________________________________________    Epitaxial Film on Semi-Insulating                                                                9.4   300    3.3 × 10.sup.16                                                                  5,900    0.31                        Substrate                 77    2.6 × 10.sup.16                                                                  11,100                               Lateral Epitaxial Film Over                                                                 Before                                                                             5.8   300    3.6 × 10.sup.16                                                                  4,900    --                          Carbonized Photo-Resist on                                                                  Transfer    77    --       --                                   Semi-Insulating Substrate                                                                   After                                                                              5.8   300    3.5 × 10.sup.16                                                                  4,800    0.42                                      Transfer    77    2.7 × 10.sup.16                                                                  9,100                                __________________________________________________________________________

The electron mobility of the laterally-grown layer were slightly lessthan the control sample, but the N_(A) /N_(D) of 0.4 indicated that itwas a very high quality. There were essentially no changes after filmtransfer.

The electrical properties of these films were comparable to the bestresults previously attainable for gallium arsenide films produced by anypreviously known growth process at these doping levels.

EXAMPLE 4

The procedures of Example 1 were employed except as otherwise noted, toproduce a separated crystalline sheet of GaAs using a single crystalGaAs substrate oriented in the [100] direction 5° off toward the nearest[110] plane. The layer separated easily leaving the separated sectionsof GaAs a little more uneven than if a [110] substrate was chosen.Although the [110] substrate is preferred, other orientations may beused depending on the application.

EXAMPLE 5

A silicon wafer is used for the substrate and it is oriented [111],which is the cleavage plane. A 500 Å thick SiO₂ layer is provided usingthermal oxidation of the entire surface of the wafer. This layer is partof the growth mask which provides a barrier against growth so that thesilicon will not grow through the silica powder which is to be used asthe other part of the growth mask. The powder is applied to the surfaceusing a mixture of 1375 Shippley Resist and silica powder with aparticle size of about 1 μm. Using conventional photolithographictechniques stripe openings are formed aligned 45° with the [110]direction which are 4 μm wide spaced 50 μm center to center in theresist layer which is about 6 μm thick. The photoresist is removed fromamong the particles of the powder by rinsing the wafer in acetone. Theparticles remain in place on the surface with the same pattern asbefore.

The following conditions were used to obtain silicon overgrowth. Theepitaxial reactor is a SiCl₄ -H₂ -HCl system. The amount of HCl wasadjusted so as to maximize lateral overgrowth. The typical growthtemperature was about 1,000° C., with a growth rate of about 0.5 μm perminute. The flow rates of SiCl₄, H₂ and HCl were 1.5 g/min, 55 cc/minand 8 cc/min., respectively. The overgrowth ratio was about 1. The Sifilms became continuous when the film was about 25 μm thick. Aseparation procedure similar to Example 1 was employed.

EXAMPLE 6

The preparation of InP films is very similar to those for GaAs. The VPEreactor uses the PCl₃ -In-H₂ process. The growth temperature is about600° C., and an overgrowth ratio of five is easily obtained. For InP,the carbonized photoresist, as in the case of GaAs, works well. Thecleavage plane in InP is [110], and it was found that lateral overgrowthcould be achieved by orienting the slits 30° clockwise from the (110)direction. This was determined using the FAN pattern. Separation wasdone by the procedure of Example 1.

EXAMPLE 7

Fabrication of a solar cell, as illustrated in FIGS. 47, 47A and 48, wasachieved, as follows.

A growth substrate was first prepared. The GaAs substrate used tofabricate the film was a single crystal with a surface oriented as closeas possible to the (110) plane. The wafer was finally thinned to aworking thickness of 16 mils from 24 mils by chemically polishing 3 milseach side with 50:50 NH₄ OH:H₂ O₂ at 53° C., Clorox polishing 1 mil fromone side, and finally etching 1/2 mil each side using 5:1:1 H₂ SO₄ :H₂O₂ :H₂ O at 27° C.

A mixture of 7:2 thinner and Shipley 1350J photoresist was uniformlyspun onto the substrate by manually increasing the rpm from 0 to 7000fairly quickly. This was followed by a heat treatment of 400° C. for 30seconds after reaching 400° C. within 2 minutes which forms the layer ofcarbonized photoresist (CPR). A thin layer of about 300 Å of pyrolyticSiO₂ was deposited at 400° C. for 20 seconds. The line openings werethen accomplished by conventional photolithographic techniques.

The line openings were opened in the SiO₂ film by a 15 second soak inbuffered HF. The photoresist was then removed using acetone. To removethe CPR a plasma etch was used in the strip mode for 5 minutes at 1 Torrusing a He/O₂ gas mixture and 50 watts of power. A light etch of 963 mlof H₂ O:7 ml H₂ O₂ :30 ml NH₄ OH for 15 seconds at room temperature wasthe final treatment prior to loading into the epitaxial reactor.

An epitaxial layer 10-11 μm thick was then grown on top of the SiO₂ -CPRstructure by growing at a temperature of 700° C. for a total of 2 hoursand 10 minutes. Under these conditions, the necessary n⁺ /p/p⁺homojunction solar cell material was produced by introducing the properdopants at the proper times.

After removing the sample from the reactor, a solar cell was fabricatedand tested in the following manner. Using conventional photolithographicprocedures, 20 finger openings were made in a photoresist film on theepitaxial film and 2-3 μm of tin was plotted in the finger openings.This first resist layer was removed and a second layer applied andpatterned in a rectangular area defining the active area of the solarcell. The top 1 μm of the gallium arsenide epitaxial layer outside theactive area was then etched away.

Cell performance was optimized by an anodizationthinning technique. Thiswas accomplished by contacting the p⁺ layer with a wire followed bymounting the sample in black wax being careful to leave the free end ofthe uncovered wire. Using an anodization solution of propylene glycol,acetic acid, and NH₄ OH, the cell was anodized to 43 volts whichproduced an antireflection coating. This mounted cell was then measuredunder a simulated sunlight source. The current was low which meant thatthe top n⁺ layer was too thick and required thinning. This wasaccomplished by immersing the sample in 1% HCl for 1 minute. The cellwas then re-anodized and re-measured. The current was still low and thecycle was repeated until the cell produced an open circuit voltage of0.943 volts and a short circuit current of 11.89 mA.

The next step was to transfer the cell from its substrate to a secondsubstrate. The cell was demounted from the black wax and the lead wasremoved. Epoxy Stycast 12 was mixed using 3 drops of catalyst to 7 dropsof resin. One drop of this epoxy was placed on the surface of the cell.Both cell and an antireflection coated 10 mil glass plate were placed ona hot plate at 60° C. and allowed to reach that temperature in about 2-3minutes. The glass with the antireflection coating facing up was thenplaced on top of the cell making absolutely sure that no bubbles form inthe epoxy and epoxy filled in between the fingers. This assembly wasleft to cure on the hot plate for 1 hour at 60° C.

To actually separate the cell, this sandwich was placed with the glassside down in an adhesive wax (Sears hot wax) on a thick 2"×2"×1/4" glassblock at 60° C. A second thick plate glass with adhesive wax was placedon top of the back side of the cell's substrate. After placing a wedgebetween the two thick glass blocks and lightly tapping with a hammer,the two halves separated at the carbon-GaAs interface. The cell mountedon its magnesium fluoride coated glass was then cleaned using acetone.

Prior to testing this separated solar cell, it was necessary to plate 2μm of gold on the back side where the cell separated from its substrate.It was also necessary to etch some GaAs away to expose a tin contact padconnecting to the contact fingers. This was accomplished by covering theback with black wax leaving an exposed area near the contact and etchingusing F.E. until the tin pad was exposed.

The cell was tested by making contact to the tin pad and the plated goldwith the following results.

    ______________________________________                                        Area (Sq Cm)        .510                                                      Spectral Type AM    1                                                         Cell Temp (°C.)                                                                            25.7                                                      Normd. Source PWR (mW)                                                                            100                                                       ISC (mA)            11.89                                                     VOC (Volts)         .943                                                      JSC (mA/Cm.sup.2)   22.99                                                     Fill Factor         .785                                                      Efficiency (%)      17.                                                       ______________________________________                                    

INDUSTRIAL APPLICABILITY

This invention has industrial applicability in the production of sheetsof crystalline material, including semiconductors, oxides and othercrystalline materials.

EQUIVALENTS

Although most of the description above is limited to gallium arsenide,silicon and indium phosphide, other semiconductors, including germanium,cadmium telluride, etc., or their associated alloys (e.g., InGaAsP,GaAlAs, HgCdTe) can also be employed in the fabrication of sheets ofcrystalline materials according to this invention. Similarly, othergrowth techniques for growing crystalline semiconductor layers overgrowth masks could be employed instead of the vapor-phase epitaxialovergrowth technique described. Instead of the AsCl₃ -GaH₂ vapor-phaseepitaxial overgrowth technique described, other growth techniquesincluding metal-organic epitaxy, molecular beam epitaxy, liquid-phaseepitaxy, vapor-phase epitaxy using other chloride systems and pyrolliticdecomposition could be employed. Similarly, other growth mask patternsother than the parallel slits specifically described could be employedand also could be used to encourage lateral growth. Also, the mask neednot be a separate material from the substrate, but might be substratematerial treated to act as a mask.

Those skilled in the art will recognize other equivalents to thespecific embodiments described herein, which equivalents are intended tobe encompased by the claims attached hereto.

We claim:
 1. A process for producing a region of single crystal siliconoverlying a region of dielectric material comprising the steps of:a.forming a region of non-single crystal silicon material which overliessaid region of dielectric material and which overlies and is in contactwith a region of single crystal silicon; and b. converting saidnon-single crystal silicon region into said region of single crystalsilicon by heating said non-single crystal silicon region with hear froma source to simultaneously melt the entire non-single crystal materialand cooling the melted material to form the region of single crystalsilicon.
 2. The process of claim 1 wherein the dielectric material is anoxide of silicon.
 3. The process of claim 2 wherein in step (b) thenon-single crystal silicon regions are melted by the heat andcrystallized.
 4. The process of claim 3 wherein the single crystalregion is converted by single crystal growth initiated from the regionof single crystal silicon.
 5. A process for producing a region ofsubstantially single crystal semiconductor material overlying a regionof dielectric material comprising the steps of:a. forming a region ofnon-single crystal semiconductor material which both overlies saidregion of dielectric material and which is in contact with a region ofsingle crystal semiconductor material; and b. converting said non-singlecrystal semiconductor region into substantially single crystalsemiconductor material by exposing the non-single crystal semiconductorregion to hear from a plurality of heat sources to simultaneously meltthe entire region of non-single crystal material and subsequentlycooling the melted region to form the substantially single crystalregion.
 6. The process according to claim 5 wherein the dielectricmaterial comprises an oxide of silicon and the semiconductor materialcomprises silicon.
 7. The process according to claim 6 wherein thenon-single crystal semiconductor region is melted by the heat andcrystallized.
 8. The process according to claim 7 wherein melting of thenon-single crystal semiconductor region is produced by heat from a laserbeam, an electron beam or a radiant energy heater.
 9. The processaccording to claim 8 wherein the non-single crystal region is convertedby single crystal growth initiated from the region of single crystalsemiconductor material.
 10. The process according to claim 5 wherein thesemiconductor material is an element of Group IV of the PeriodicClassification of the elements and the non-single crystal material is incontact with the said region of single crystal semiconductor material.11. A method according to claim 1 including an additional step offabricating a device from the single crystal semiconductor material. 12.A method according to claim 1 wherein the non-single crystal material isdeposited on the dielectric material in the presence of selectedimpurity dopants to form layers of different conductivity type in thesingle crystal material.
 13. A method according to claim 1 whereinplural sheets of single-crystal material are formed and are bondedtogether by an electrically insulating or electrically conducting layerto form a device in which the sheets are electrically decoupled orelectrically connected, respectively.